1. Technical Field
The invention relates to integrated circuits. More particularly, the invention relates to a method and apparatus for attaching a cooling structure to an integrated circuit.
2. Description of Prior Art
Heat-sink-attachment and thermal-interface-design options are based, on performance considerations, in addition to cost effectiveness. These performance considerations include functional considerations such as thermal properties, and reliability considerations such as mechanical and environmental properties. In addition, the ease and cost of assembly and disassembly of the heat sink (HS) attachment structure are important.
A typical state of the art attachment of a HS or a heat spreader to the backside of a high-power chip physically interconnects the HS structure to the system's substrate structure, namely the printed circuit board (PCB). Such a design approach is used when there is a need to produce a high enough pressure at the thermal interface between the HS and the chip. Such pressure is often needed for a satisfactory thermal performance of the interface.
Also known in the art are attachments of an HS to a package substrate (PS), rather than to the PCB. Designs of this type are not intended and, in many cases, are not even supposed to produce high pressure at the HS/chip interface. Such HS-to-package attachment designs in the current art are acceptable if a relatively low pressure, for example a pressure in the range of 5-20 psi, can ensure a satisfactory thermal management of the integrated circuit (IC) device.
However, it is becoming a more frequent requirement in the industry that a high interfacial pressure of, for example in the range of 70 psi or higher, is needed to produce and control a satisfactory thermal contact. This occurs for example, in the case where a carbon nano-tube (CNT) based HS is used and the tips of the CNTs require high pressure to bend sufficiently and provide the necessary thermal contact. Such a HS is discussed in U.S. patent application Ser. No. 10/925,824, System and Method Using Self-Assembled Nano Structures in the Design and Fabrication of an Integrated Circuit Micro-Cooler, assigned to common assignee and which is herein incorporated by reference thereto for all that it contains. In such case the HS is typically attached to the PCB. This is usually done by using screw-based elements, with or without springs, or flat-spring-based structural elements. This approach can produce a very high pressure at the HS/chip interface providing the necessary pressure to achieve the thermal interface required. However, this approach does suffer from the shortcomings that are discussed below.
As shown in FIGS. 1a and 1b chip 110, connected in a flip-chip (FC) position, is soldered to a package substrate 140 through solder joints 120. A metal frame 130 mounted on top of the substrate 140 further secures the chip 110. The objective of such a reinforcement is to increase the flexural rigidity of the substrate 140 so that it does not bend as a result of the elevated temperature, typically is the range of 220° C. to 280° C. or so, applied to the system during the reflow soldering process. As is known, such a process is an essential part of the technology that is currently used to surface-mount IC packages on PCBs. The solder joints 120 are coupled to ball grid array (BGA) solder joints 150 through the substrate 140, the BGA solder joints 150 being soldered to the PCB 160. The BGA material is typically the most vulnerable part of the BGA package structure. Using prior art solutions, when a screw-based design is used to mount an HS on a package that is further attached to the PCB 160, it is not only the thermal interface that experiences elevated pressure, but also the BGA solder joints 150. This circumstance, favorable from the standpoint of the thermal performance of the device, can have a detrimental effect on the reliability of the BGA solder material, both on a short-term basis because of the excessive static overload, and on a long-term basis because of the significant mechanical loading added. During the system manufacturing, for example, during the reflow soldering process, and operation, for example, power cycling conditions, such a mechanical loading superimposes the thermally induced loading. The thermally induced loading is caused by the change in temperature in the structure in question which is fabricated of dissimilar materials. In addition, temperature gradients are experienced. A significant mechanical pre-stressing may worsen the mechanical performance. That is, the adhesive and/or cohesive strength of the BGA solder material is affected. In some cases, the elevated tensile forces that are applied to the PCB 160 as a result of the mounting of a HS to it can lead to local mechanical, also referred to as physical, damage of the PCB 160 and can result in electrical opens and/or shorts.
Therefore, due to the limitations of prior art solutions, it would be advantageous to provide an HS mounting structure, such that the BGA solder joints and the PCB do not experience the high pressure applied at the thermal HS/chip interface.